diff --git a/src/Entity/Facet.php b/src/Entity/Facet.php
index e9163f14e1297abd7565391939f5ab47ff1155c6..f4cf01bb76df8f0b9782db5303989776137f7fa6 100644
--- a/src/Entity/Facet.php
+++ b/src/Entity/Facet.php
@@ -372,7 +372,7 @@ class Facet extends ConfigEntityBase implements FacetInterface {
   /**
    * {@inheritdoc}
    */
-  public function setWidget($id, array $configuration = NULL) {
+  public function setWidget($id, ?array $configuration = NULL) {
     if ($configuration === NULL) {
       $instance = $this->getWidgetManager()->createInstance($id);
       // Get the default configuration for this plugin.
@@ -410,7 +410,7 @@ class Facet extends ConfigEntityBase implements FacetInterface {
   /**
    * {@inheritdoc}
    */
-  public function setHierarchy($id, array $configuration = NULL) {
+  public function setHierarchy($id, ?array $configuration = NULL) {
     if ($configuration === NULL) {
       $instance = $this->getHierarchyManager()->createInstance($id);
       // Get the default configuration for this plugin.
diff --git a/src/FacetInterface.php b/src/FacetInterface.php
index b92b5cad3c51296b72687a5049c35d9db00adccc..c44803eafbe94ac56046e17441ab105be2c96179 100644
--- a/src/FacetInterface.php
+++ b/src/FacetInterface.php
@@ -18,7 +18,7 @@ interface FacetInterface extends ConfigEntityInterface {
    *   (optional) The facet widget plugin configuration. If missed, the default
    *   plugin configuration will be filled.
    */
-  public function setWidget($id, array $configuration = NULL);
+  public function setWidget($id, ?array $configuration = NULL);
 
   /**
    * Returns the facet widget definition.
@@ -47,7 +47,7 @@ interface FacetInterface extends ConfigEntityInterface {
    *   (optional) The facet hierarchy plugin configuration. When empty, the
    *   default plugin configuration will be used.
    */
-  public function setHierarchy($id, array $configuration = NULL);
+  public function setHierarchy($id, ?array $configuration = NULL);
 
   /**
    * Returns the facet hierarchy definition.